496 CHEMICAL ENGINEERING
and from there on, every additional 25°C. up to 1,500°C. The rise in temperature above 900°C. should be at the rate of from 20 to 25°C. per hour.
Upon removal from the furnace the briquettes are either covered with hot sand or placed in a small furnace kept at a red heat so as to avoid checking or cracking on cooling. When cooled, the specimens are weighed and then immersed in clean water, boiling under a partial vacuum so that the liquid will penetrate into all the open and communicating pores. It is then possible either to make a simple absorption or a porosity determination. For general work the former will suffice. If then the absorption data are plotted against the temperatures either in degrees or expressed in cone numbers a curve will be obtained which shows the firing behavior of the clay very clearly. The vitrification temperature will be that point at which the absorption falls to a minimum, usually a value not exceeding 1 per cent. The overfired condition is indicated by a marked rise in the absorption due to the formation of a spongy or vesicular state. In many of the published tests, the porosity value is plotted against the temperature in place of the absorption. The softening point determination has already been described in previous paragraphs.
There are thus three fixed points to consider in the testing of fireclays or for that matter in the examination of refractories in general, viz., the vitrification temperature at which the mass becomes dense and practically impervious, the point at which it becomes overfired and vesicular and the softening temperature. The difference between the vitrification and overfiring temperature constitutes the vitrification range which is of considerable technical importance from the manufacturing standpoint.
The results of physical tests made upon a number of refractory bond clays are given in Table 2.
With the increase in silica and the presence of varying amounts of fluxes the coal-measure fireclays tend to become dense or vitrify at higher temperatures than the bond clays even though the latter possess a higher ultimate refractoriness. It is evident, however, that there are also many clays of this type maturing at a low temperature, as for instance the carboniferous clays from Brazil, Ind.
For high-grade clays of any kind it is necessary that the total fluxes be as low as possible. From certain work done by the writer it appears that more than 0.22 molecular equivalent of fluxes is undesirable with a silica content up to Al2Os-3SiO2. With a higher content of silica the permissible RO content decreases. Thus a clay of the composition 0.17 RO.A12O3.4 Si02 is apt under the compression conditions met with to fail at furnace temperature.
Siliceous Clays.—The necessity of dividing the refractory clays into two classes, low and high in silica arises from the fact that the two types behave quite differently in use. The division line between the two kinds of materials has been arbitrarily fixed at a silica content of 70 per cent. It is obvious that the gradation from one class to the other is not an abrupt one, but that the properties of the first gradually merge into those of the second. These clays thus consist of clay substance plus a considerable amount of free silica as quartz sand.
There are several characteristics by means of which the two types may be distinguished. As a class, though with certain exceptions, the siliceous clays possess less plasticity, bonding power and strength in the dry state than the more aluminous ones, and also a low drying shrinkage. In the firing process they usually show no decided decrease in porosity and hence remain porous. As a result, their firing shrinkage is very low and not infrequently nilj due to the expansion of the quartz.
Finally, siliceous clay refractories upon reheating in use are free from the tendency to shrink characteristic of the high clay materials. In addition they are able